High frequency generator circuits such as voltage controlled oscillators (VCOs) are used as local oscillators (LOs) for mixing signals in order to transmit and/or receive information, e.g., over a wireless or wired channel. With higher transmission frequencies (e.g., above 1 GHz), it is difficult to generate a “clean” high frequency signal. Conventional methods of LO generation have large harmonics and spurs that can desensitize the receiver and create spectral compliance challenges for the transmitter. The noise on the LO can mix with blockers and desensitize the receiver. It can be even more of a challenge when the various transceivers supporting different frequency bands are implemented in a single chip. For example the unwanted spectral tones of a local oscillator can interfere with a receiver section of another transceiver.
Direct conversion transmitters are desired for low cost, small size and reconfigurable modulation bandwidth. The frequency generator, the VCO, phased locked using a PLL to a crystal oscillator is needed for up-conversion. A VCO running at the transmit frequency, however, can suffer from frequency pulling by the transmit amplifier. For higher-power transmit amplifiers, the high power section may be separated and shielded (e.g., different chip in a metal-enclosure) from the VCO (or VCO could be implemented with a separate shielded chip) circuit to inhibit interference from the high power transmitter back to the sensitive VCO circuit. Unfortunately, it may be less desirable to use separate chips due to cost reasons, but with present approaches, if implemented in a single chip, the VCO may be detrimentally interfered with by the higher power section. Some transceiver solutions have involved splitting up the conversion into several stages so that the higher power stage like a transmit power amplifier is at a different frequency than the VCO used in the local oscillator. But this requires more than one VCO and possibly more than one PLL. The other so called direct conversion solutions generate the LO signal indirectly from a VCO that is offset from the carrier frequency. For example, if a 6 GHz transmission frequency is desired, a VCO may be used to generate a 4 GHz signal, which can be divided to obtain a 2 GHz component. The 4 GHz and 2 GHz signals are then used to attain the desired 6 GHz LO signal. Unfortunately, this requires additional circuitry (mixers, etc.). Because of frequency mixing the LO spectrum may contain unwanted components at 2 GHz, 4 GHz, 8 GHz etc in addition to desired 6 GHz components, requiring in some cases, tuned circuits to reduce these levels. These circuits as well as the mixer used for LO generation also need large inductors, which consume more power and chip area and make the solution expensive. Accordingly, new approaches are desired.